The present invention relates to the art of magnetic resonance imaging. It finds particular application in conjunction with shifted interleaved multi-volume acquisition for 3D fast spin echo (SIMUVA) and will be described with particular reference thereto.
The Fast Spin Echo (FSE) imaging technique is a state-of-art technique, widely used in magnetic resonance imaging (MRI) applications. The technique provides a good tradeoff between shorter imaging time and distinct soft tissue contrast. Based on its successful application in 2D acquisition mode, 3D FSE was developed in order to cover a larger volume within a given longer repetition time, to improve signal-to-noise (SNR) and spatial resolution in the slice direction. For larger volumes and some applications, a series of 3D slabs are combined to form the volume image.
However, the 3D FSE technique suffers a problem of discontinuous boundaries (i.e., non-uniform signal intensity modulation) or venetian blind artifacts across the volume coverage, particularly the slab boundaries. Venetian blind artifacts involve signal intensity oscillations along the z-direction which degrades image quality and leads to falsified morphology and tissue contrast. The main causes of venetian blind artifacts in 3D FSE are i) imperfect RF pulses which result in amplitude fall-off and aliasing along the z axis at the edge of each volume, ii) inconsistent repetition time leading to non-uniform steady-state of spins in the overlapped regions, and iii) physical fluid motion in vivo.
Unlike the case in 3D Magnetic Resonance Angiography (MRA) in which only one RF pulse is applied to excite spins, the problem caused by imperfect RF pulse profiles is more complicated in 3D FSE because different echoes experience different RF trains. This means the imperfection of RF pulses (amplitude and phase behavior) might be accumulated with the echo train length (ETL) which makes it difficult to model such imperfections from echo to echo. That is, it becomes difficult to compensate for such effects.
The conventional solution that addresses this problem is to use slab overlap and average or blank the overlapping portion. See for example U.S. Pat. No. 5,167,232 of Parker et al. Analogous to that in 3D multiple slab/volume MRA, this strategy is straightforward, passive, and easy to implement. However, the slab overlap technique mitigates venetian blind artifacts only if the artifacts are caused by the imperfection of RF pulses alone.
Slab (volume) overlapping means an oversampling along z-axis is performed (typically defined by sampling ratio). The oversampled slices will be discarded or blanked due to phase aliasing and amplitude fall-off. Unfortunately, the oversampling strategy introduces slab overlapping regions in which the steady-state of spins is different with that of non-overlapping volumes, and is very hard to remove. The steady state of spins in the overlapped regions is dependent on slab overlap, the total number of volumes (even or odd), and the acquisition order (the particular interleaved acquisition technique for multi-volume 3D FSE).
When oversampling is applied, there are overlapped sub-volumes across two adjacent volumes experiencing non-uniform steady-state which contribute to the venetian blind artifacts. Unlike 3D MRA in which at 50% oversampling venetian blind artifacts can be removed; in 3D FSE the more oversampling, the more severe the venetian blind artifacts. Also, the shorter the repetition time, the more severe the venetian blind artifacts. Thus, using a slab or volume overlap strategy is contradictory in the sense that it mitigates the problem caused by imperfect RF pulses, but it exaggerates the non-uniform steady-state effect across the volume coverage. That is, more overlapping slices mitigates the imperfection of RF pulses but enlarges the non-uniform steady-state region, consequently expanding the dark bands of venetian blind artifact. In addition, slab overlap suffers from a time penalty thereby slowing down the scanning. This means longer imaging time is needed to cover the same volume. For these reasons, a larger slab overlap (oversampling) technique is a nonrobust and time-consuming solution.
Accordingly, it has been considered desirable to develop a new and improved shifted interleaved multi-volume acquisition (SIMUVA) technique which meets the above-stated needs and overcomes the foregoing difficulties and others while providing better and more advantageous results.